1. Field of the Invention
The present invention relates to a radiation detector, and more particularly to a radiation detector capable of discriminating differences in electric charge quantity or mass number between an alpha ray and a beta ray, an alpha ray and a gamma ray, an alpha ray and a proton ray or an ion beam.
2. Description of the Related Art
Discriminating methods for detecting radiation by using a scintillator are well-known. These discriminating methods enable distinguishing a neutron from a gamma ray, an alpha ray from a beta (or the gamma) ray or the like. Here, a method of discriminating between the neutron and the gamma ray is called xe2x80x9czero-crossing methodxe2x80x9d, and this method employs a liquid scintillator in order to discriminate radiation by time. The zero-crossing method applies the fact that there exists a time difference between double-differentiated waveforms thereof on zero axis due to difference of rise times.
As for a method of discriminating the alpha ray from the beta (or the gamma) ray, a so called xe2x80x9crise time discriminating methodxe2x80x9d is often employed. In this method, a delayed signal and an attenuated amplitude signal are obtained from one signal, and a time difference between these signals is assumed to be in proportion to the rise times thereof, thereby converting to pulse height signals. This method, employing the rise time information for discriminating the alpha ray from the gamma ray, is known as a phoswich detector. Here, the xe2x80x9cphoswich detectorxe2x80x9d is named after a xe2x80x9csandwich type phosphor detectorxe2x80x9d.
FIG. 1 shows a basic structure of the phoswich detector.
In FIG. 1, a photo detector (radiation sensing part) 30 includes a powdery ZnS(Ag) scintillator 30a and a plastic scintillator 30b thinly spread on the ZnS(Ag) scintillator 30a. An alpha ray is mainly detected by the ZnS(Ag) scintillator 30a, and a beta ray is mainly detected by the plastic scintillator 30b. The scintillation decay time of the ZnS(Ag) scintillator 30a is rather long, such as microseconds, while the scintillation decay time of the plastic scintillator 30b is rather short, such as ten nanoseconds. A photomultiplier tube 16 is equipped with a preamplifier having an integrator whose time constant is longer than the scintillation decay time. Therefore, if the output from the photo detector 30 is applied to the photomultiplier tube 16, the rise time of the output from the photomultiplier tube 16 indicates information as to the scintillation decay time of each scintillator.
The output from the photomultiplier tube 16 is amplified by a fast amplifier 32, and is then analyzed by a time analyzer 33. The output from the time analyzer 33 is given by a pulse signal whose pulse height is proportional to the rise time. Therefore, the scintillations of the ZnS(Ag) scintillator 30a and the plastic scintillator 30b can be discriminated by employing a pulse height discriminator and a counter, or a multi-channel pulse height analyzer.
However, conventional methods need supplemental devices other than general detectors of scintillation to discriminate radiation. Therefore, this requires additional cost for the supplemental devices.
Further, these discriminating methods include attenuation of signals caused by differentiation in the zero-crossing method and by intended actions in the rise time discriminating method, and these attenuations are known to affect S/N ratio. Therefore, conventional methods do not have enough performance to detect and discriminate radiation.
The present invention has been made in view of the above-mentioned circumstances and is intended to solve the above-mentioned problems. In particular, the object of the present invention is to provide a radiation detector having a simple and inexpensive structure, and capable of discriminating radiation with high efficiency.
The present invention provides a radiation -detector, including: a scintillator that scintillates depending on radiation and generates a first output signal and at least one second output signal after the first output signal, wherein said at least on second output signal or signals vary the signal pattern thereof with a kind of rays in radiation; and a discriminator that discriminates a kind of rays in radiation based on the first signal and the second signals.
The scintillator may have a multiple scintillation characteristic. The scintillator may include Gd2O2S (GOS) or Y2O2S (YOS). The scintillator may be activated by europium (Eu) or terbium (Tb).
The number of the second output signals while a beta ray is detected may be larger than when an alpha ray is detected.
The discriminator may further include: a time gate generator that generates a time gate having a certain time width as an input of the first signal as a starting point; a counter that counts the number of the second signals in the time gate; and a multiple scintillation discriminator that compares a counted number in the time gate with a predetermined value thereby outputs a discriminated result.
The discriminator may further include: a time gate generator that generates a time gate having a certain time width as an input of the first signal as a starting point; a counter that counts the number of the second signals in the time gate and divides the time gate proportionally in order to count the number of signals in each of the divided time gates; and a multiple scintillation discriminator that compares a counted number in the divided time gate with a predetermined value thereby outputs a discriminated result.
The discriminator may further include: a multiple time gate generator that generates time gates having a certain time width as inputs of the first signal and the second signal as starting points; a gate width discriminator that compares width of the time gates with a predetermined value thereby outputs a discriminated result.
The discriminator may further include, an amplifier that includes a resistor and a condenser coupled in parallel to each other, the amplifier charges a current obtained by photoelectric conversion of the first signal and the second signals to the resistor and the condenser, and amplifies charged/discharged voltage generated on terminals as an integrated waveform; and an integrated pulse height value discriminator that compares pulse heights of the waveforms with a predetermined value thereby outputs a discriminated result.
The discriminator may further include: a time gate generator that generates a time gate having a certain time width as an input of the first signal as a starting point; and a time interval discriminator that compares a time interval of the second signals generated in the time gate with a predetermined time interval thereby outputs a discriminated result.
The discriminator may further include: a time gate generator that generates a time gate having a certain time width as an input of the first signal as a starting point; and a pulse height value discriminator that compares a pulse height value of a pulse signal of the first signal with a pulse height value of a pulse signal of the second signal thereby outputs a discriminated result.
The discriminator may further include: a time gate generator that generates a time gate having a certain time width as an input of the first signal as a starting point; a counter that counts the number of the second signals in the time gate; a discriminator that compares a counted number in the time gate with a predetermined value thereby outputs a discriminated result; and a dead time compensator that compensates the discriminated result by considering a dead time of the counter.
The scintillator may include a first material capable of providing a multiple scintillation and a second material capable of radiating a charged particle. The scintillator may include a first material capable of providing a multiple scintillation and a third material capable of providing a non-multiple scintillation.
The scintillator may include a material capable of providing a multiple scintillation and a material capable of providing a non-multiple scintillation, and the multiple scintillation and the non-multiple scintillation may be individually detected so that the first output signal and the second output signal are respectively generated.
The present invention further provides a scintillator including a material having multiple scintillation characteristics and scintillates depending on radiation and generates a first output signal and at least one second output signal after the first output signal, wherein said at least one second signal varies the signal pattern thereof with a kind of rays in radiation.
The present invention further provides a method of discriminating a kind of rays in radiation, including: providing a scintillator that scintillates depending on radiation and generates a first output signal and at least one second output signal after the first output signal, wherein said at least one second output signal varies the signal pattern thereof with a kind of a ray in radiation; and discriminating a kind of rays in radiation based on the first signal and said at least one second signal.